Detection and tracking of targets by sensor systems have been the subject matter of a large number of practical applications. Sensor systems designed for this purpose may use propagating wave signals, such as electromagnetic or acoustical signals. Some sensor systems, such as radar and active sonar systems, are designed to receive reflections of a transmitted signal generated by an appropriate transmitter, and determine the presence of objects by analyzing the transmitted and the reflected signals.
Active sensor systems are generally used for detection of scattering objects. In the presence of a scattering object, the transmitted signal arrives to the receiving sensor system with a certain time delay, which is related to the range of the scattering object (i.e., the distance to it). Also, if the scattering object is moving, the reflected signal exhibits a spectral shift that is known as a Doppler shift. The Doppler shift is related to the relative velocity of the object with respect to the sensor system. In order to provide an example of a received signal in an active sensor system, a simulation has been conducted for a radar system that transmits a phase-coded radar signal as shown in FIG. 1A. In this simulation, the transmitted signal reflects back from an object, at a 12 km range, moving with a velocity of 400 m/s towards the radar system. The reflected signal is received by the radar antenna and down-converted by a conventional analog receiver system, such as shown in FIG. 2. The output of the analog receiver system is shown in FIG. 1B, where the effects of the object range and velocity are seen as a delay and an additional frequency modulation of the received signal, respectively. These two prominent effects of the received signal can be more readily observed on the cross-ambiguity function of the transmitted and received signals, which is defined as:Ars(τ, v)=∫r(t+τ/2)s*(t−τ/2)exp[j2πvt]dt,where s(t) is the transmitted signal and r(t) is the received signal. For the transmitted and received signal pair shown in FIGS. 1A and 1B, respectively, the magnitude of the cross-ambiguity function is illustrated in FIG. 1C as a 3-Dimensional plot. In FIG. 1D, the contour plot of the same cross-ambiguity function is provided. Since it is easier to visualize the structure, contour plots of the cross-ambiguity function are more commonly used in practice. As seen in FIG. 1D, the peak of the cross-ambiguity function is located at the corresponding delay and Doppler shift caused by the scattering object. This observed correspondence between the peak location of the cross-ambiguity function on one hand, and the position and the velocity of the scattering object on the other is a general relationship, which holds true in most cases where there is no or little noise at the receiver.
In the case of a noisy reception of the reflected signal, the peak location of the cross-ambiguity function still provides a reliable estimate of the delay and the Doppler shift caused by the scattering object. Therefore, in accordance with the present invention it is possible to detect the presence of one or more scattering object by finding the peak locations of the cross-ambiguity function and comparing them with appropriately chosen threshold levels. Those peaks that exceed the thresholds can be identified as scattering objects, and the locations of the peaks will provide the corresponding delay and Doppler shift information at the same time. Such peaks of the cross ambiguity function may be computed by calculating the entire cross-ambiguity function and then examining it for peaks. This computation is complex and processor intensive.
The main objective of the present application is to provide an efficient and low-cost system and method that can reliably detect scattering objects and estimate both their delay (i.e. distance to the radar) and their Doppler shifts at the same time, without actually computing the entire cross-ambiguity function. One such method is known in the art and is disclosed in U.S. Pat. No. 6,636,174, incorporated herein by reference. To detect a target in accordance with the U.S. Pat. No. 6,636,174 patent, two projections at different angles of the cross-ambiguity function are computed. A projection is a collection of integrals (or summation of samples) taken over uniformly spaced paths perpendicular to the axis of the projection in the Doppler shift/time delay plain at a selected angle. The angle of the projections would be pre-determined by the environment, i.e. advanced knowledge of the approximate velocity and direction of the scattering objects.
Another method for detection of objects is disclosed in U.S. Pat. No. 7,218,274, incorporated herein by reference. To detect a target, in accordance with the methods disclosed in U.S. Pat. No. 7,218,274, a projection is computed first and then if a peak on this projection is detected, a slice passing through the peak of the projection is computed to localize the peak of the cross-ambiguity function, where a slice is a plurality of samples of the cross-ambiguity function lying over a line or line segment. The angle of the projection would be pre-determined by the environment, or, alternatively, a plurality of projections at different angles may be calculated and the one with the highest peaks is chosen as the basis for further computations. All projections may be calculated without sending and receiving additional pulses.
In the present application, more efficient methods of detection of targets are disclosed.